The invention relates to a method for joining microstructured component layers that are suitable for manufacturing microstructure components and that comprise metals, in particular aluminum and/or aluminum alloys, copper/copper alloys, and/or high-grade steels and furthermore relates to a microstructure component encompassing a stack of microstructured component layers that are joined to one another and that comprise metals and/or metal alloys, in particular aluminum and/or aluminum alloys, copper and/or copper alloys, and/or high-grade steels.
Microstructure components, that is, micro-(μ)-reactors, μ-mixers, and μ-heat exchangers are already employed in chemical process technology and in research and development projects. The first industrial processes have already been realized. For instance, the Clariant Company, Switzerland, jointly with the CPC Company, Germany, installed a pilot system for manufacturing two commercial azo pigments and tested the continuously-working method. The results of this method are that up to 149% greater color intensity, brighter and more transparent particles, etc. were attained (CHEmanager, May 2002). The PAMIR study (PAMIR: Potential and Applications of Microreaction Technology—A Market Survey, Mainz Institute of Microengineering, GmbH and YOLE Développement; 2002) provides a survey of the potential for microstructure technology and current industrial applications for microreaction technology. For further information on research and development projects, refer to Ehrfeld, et. al., “Microreactors,” Wiley VCH, 2000 and to the proceedings of the “Microreaction Technology” conference, which has been held annually since 1997 (IMRET 1 through 6, 1997 to 2002).
Compared to conventional reactors and heat exchangers, microstructure components are characterized by excellent advantages:                1. Nearly isothermic method        2. High surface/volume ratio        3. Heat transfer improved by orders of magnitude, i.e., extremely compact high-performance heat exchangers for a wide variety of applications, e.g., for fuel cells, air conditioning in automobiles and airplanes, coolers for electronic components with high heat development        4. Extremely compact construction        5. Highest degree of system integration        6. Improved process controls        7. Very high safety standard, even during highly exothermic reactions.        8. Improved environmental protection        
Microstructure components in general comprise a stack of thin metal sheets that are characterized by fine structures. Joining the structured sheets results in components with very fine channels, whereby the cross-section is typically less than 1 mm2. The sheets can be structured using dry etching methods, using wet chemical deep etching, or using mechanical microfinishing.
Normally the structured sheets are provided with a cover plate and a bottom plate and are joined into one compact component. When designed appropriately, the components attain maximum heat exchange or power exchange with minimum component volume, whereby the flow conditions in the component are adjustably defined and nearly isothermic conditions can be attained in the microchannels.
μ-Reactors, μ-heat exchangers, and μ-mixers on the market are generally made of high-grade steels because of the production engineering described in the following. In addition, the microstructure components with this production engineering cannot be employed in many fields of application, first because cost-efficient manufacture for large series and mass production is not possible and/or large numbers cannot be achieved at all or can only be achieved at great cost, and second the manufacturing method must lead to components that satisfy the following technical requirements:                1. Sealed, both between microflow channels and to the environment;        2. Pressure resistance/strength;        3. Corrosion resistance to media used;        4. Temperature resistance;        5. Free geometrically well defined fluid channels, that is, channels that are free of interfering residues from the manufacturing process.        
In the prior art, thermodiffusion welding exclusively is used to join the stacked metal films for manufacturing metal μ-reactors, μ-mixers, and μ-heat exchangers. The stack of sheets to be joined, which is made of individual microstructured films, is welded to one another in a high vacuum under high pressure at high temperatures via interdiffusion. The advantage of this method is that monoliths, that is, component cores made of a uniform material, are produced. In order to provide sufficient interdiffusion, very high demands are placed on surface quality (roughness, purity, shape/planarity) during thermodiffusion welding to the components to be joined. This leads to restrictions in material selection and to expensive process conditions and material preparations. This is particularly dramatic in the case of aluminum and its alloys since the high oxygen affinity of the aluminum materials leads to the formation of oxide layers, even when work is performed under good vacuum conditions. In the past this problem has led to high reject rates during production so that this joining method is not economical for industrial applications. The subsequent housing process (forming a bond between the housing, including terminals, and the reactor body), typically by electron-beam welding, is subject to this restriction to an even greater extent since material combinations within a component are very difficult and very high local heat development can result in leaks in the diffusion-welding seams on the reactor body. Disadvantages of thermodiffusion welding are consequently the following listed complex manufacturing conditions: high vacuum, preferably very high joining temperatures (˜1000° C.), long standing and processing times, and restrictions with respect to basic materials and material combinations. The resultant costs of such products drastically limit their use. The prices of such components are currently therefore between a few hundred and a few thousand Euros (e.g., in accordance with price lists from the Mainz Institute of Microengineering).
Soldering/brazing processes have the disadvantage that the joining coating comprises other metals than the stacked films. However, these methods offer fundamental advantages in terms of costs. Although soldering/brazing methods have been suggested repeatedly as a joining technique for microstructure components, in the past it has not been possible to use soldering/brazing methods for industrial manufacture of μ-components since the requirements during the manufacture μ-components are very stringent:                1. No solder/brazing material may run into the channels during the melting process so that the channels become stopped up.        2. Work must be performed without any flux at all since flux residue cannot be removed from the finished component, or is extremely difficult to remove;        3. The solder/brazing coating must be very thin, homogeneous, uniformly distributed, and still error-free due to the small dimensions and complexity of the structure.        
Humptson (Humptson, G. J., Jacobson, D. M., “Principles of Soldering and Brazing,”, 4 (2001), ASM International, The Materials Information Society, ISBN 0-87170-462-5) describes Transient Liquid Phase Bonding in greater detail. This is a diffusion soldering/brazing method in which one or two solder/brazing coatings are produced between the parts to be bonded and the bond is heated to a temperature above the melting point of the solder/brazing material. The bond is heated over a lengthy period in order to make possible interdiffusion of the solder/brazing metals and the metals of the base materials. A eutectic melt of the two metals or alloys can also be formed if two different solder/brazing metals or alloys are used. It is assumed that copper, silver, or gold is used as solder/brazing material for most of the solder/brazing processes. A typical example of a solder/brazing material for use in a diffusion method is the copper/tin alloy system.
CH 690 029 A5 describes a fusible coating made of at least two layers on a substrate, in particular that can be used for solder/brazing. The manufactured substrates can be used advantageously for gas- and liquid-tight joints in watch housing parts using brazing. For manufacturing the soldering/brazing coating, two partial coatings are deposited electrolytically. During the brazing process the coatings form a eutectic melt that has a melting point at the temperature that is usual for brazing, that is, in general a melting point above 450° C. and below approx. 1000° C. For the solder/brazing coating, gold and nickel are indicated as components in a ratio of approximately 7:3 for brazing white gold, stainless steel, and titanium and titanium alloys with high portions of titanium. Other metal combinations that can be used for brazing are for instance manganese and copper as well as copper and silver. If a gold/nickel coating is applied to stainless steel, preferably a coating of gold is first deposited on the substrate.
Furthermore, EP 0 212 878 A1 describes a method for manufacturing a heat exchanger in which the flow channels for the heat medium are formed in steel plates. The steel plates are bonded to one another using diffusion bonding.
As discussed in the foregoing, microstructures currently on the market are overwhelmingly made of high-grade steel due to production constraints. But a particular challenge with respect to producing microstructure components is the use of aluminum/aluminum alloys, especially in terms of joining. In the past no microstructured components that satisfied the aforesaid technical requirements could be made from aluminum materials. For this reason this problem should be discussed in greater detail at this point:
The low density (2.7 g/cm3) of aluminum and its favorable strength properties make possible optimum shape and light weight construction and thus substantial savings in weight. This reduction in the mass of the component is extremely important for applications in vehicle design and aerospace engineering. In addition to combining light weight with great strength, aluminum has a highly electropositive character and correspondingly has a high affinity to atmospheric oxygen. In contrast to easily corrosive steel, aluminum is resistant to air due to the formation of a coherent thin oxide coating, since this prevents further attack, and thus corrosion, by oxygen.
It is precisely this protective coating responsible for aluminum's high corrosion resistance that prevents successful bonding of aluminum layers or parts during the manufacture of microstructure components or that leads to high reject rates and must therefore absolutely be removed prior to the joining process. Used for this for instance during brazing are fluxes that normally melt at temperatures of approx. 570° C. and dissolve the Al oxide coating. Use of flux should be avoided when possible since there are substantial disadvantages associated with its use, such as for instance environmental pollution, corrosion, undesired reactions between flux and for instance alloy constituents of the base material, and additional costs associated therewith. In addition, large surface areas can often be bonded together only inadequately when flux is added since build-ups can occur due to incomplete run-off of the flux during the joining process and this can cause the probability of corrosion to rise sharply. For these reasons other methods have been developed in which joining can be performed without using flux. None of these methods in the past has been able to be employed to successfully manufacture microstructured components, however.
Currently intensive research is being devoted to resolving the problems associated with joining microstructured components made of aluminum/aluminum alloys because of the great importance of aluminum materials.
In general soldering/brazing is already employed commercially as a thermal joining method in a vacuum or in an inert gas atmosphere. However, the films or pastes used when soldering/brazing the microstructure easily lead to the microchannels becoming stopped up, so that this method is not suitable for use as a joining method for microstructure components. Furthermore, the addition of flux normally used during soldering/brazing can lead to corrosion of the joints since the flux accumulates in the solder/brazing gap of the microchannels. In addition, flux is very unfriendly to the environment and its effects cannot be minimized without taking complex and expensive steps to purify waste water and exhaust air. Also, there can be undesired reactions between the flux and the alloy additives so that the bond desired between the joining partners does not have the desired properties in addition, when manufacturing catalyst-coated microreactors the use of flux can lead to deactivation of the catalyst used.
For joining aluminum and/or aluminum alloys, US 2002/0012811 A1 for instance provides that the material on the surfaces to be joined are first pretreated and then a metal coating containing nickel that also contains bismuth is electrolytically applied to the pre-treated surfaces. The joining process can be performed without flux. The nickel/bismuth-coated aluminum materials can be used for manufacturing heat exchangers.
Steffens, H.-D., Wilden, J., Möwald, K., “Use of ion-plated diffusion barriers and soldering/brazing systems when soldering/brazing steel/light metal compounds,” (DVS, 166, 94-98 (1955)) provides that eutectic aluminum base solder/brazing can be used for flux-free soldering/brazing. Prior to applying the aluminum base solder/brazing, a coating of titanium as an adhesive agent and a nickel coating as a wettable surface, which also acts as a diffusion barrier, is applied by means of TiNi ion plating.
Petrunin, I. E., “Contact Reaction Soldering/Brazing,” Handbook of Soldering/Brazing Technology, Verlag Technik GmbH Berlin, 1990, provides techniques for soldering/brazing aluminum and its alloys. According to it, aluminum can be soldered/brazed without flux in a gas atmosphere using the contact reaction method without using surface protective coatings. Silicon, copper, or silver can be used for solder/brazing material; they are applied to the aluminum surface electrolytically, by vapor-deposition, or by screen printing. Surface protective coatings, for instance coatings made of aluminum, copper, nickel, silver, zinc, etc., can also be used if no flux is to be used. The coatings can also be formed electrolytically or chemically.
DE 197 08 472 A1 describes a manufacturing method for chemical microreactors in which the fluid channels are formed in individual planes. The individually manufactured planes are then collected into a stack and joined securely to one another, for instance by soldering/brazing. The individual substrates can comprise metals/metal alloys. For joining the individual layers, one method cited is a brazing method using solder/brazing containing silver, and another method is cited in which first tin coating is deposited and then a bismuth coating is deposited thereupon. In this case a low fusing eutectic mixture forms at the interphase when the coatings are heated and further tempering forms a bond that has an elevated melting point.
For manufacturing a heat exchanger for a Stirling motor, in accordance with Bocking, C., Jacobson, D., Bennet, G., “Layer manufacturing of heat exchange elements using photochemical machining, electroplating, and diffusion brazing”, Trans. IMP, 2000 78(6), 243-246, copper sheets are used in which fluid flow channels are formed using chemical etching. The sheets are joined to one another using a diffusion brazing method. Tin is electrolytically deposited on the copper sheets, and the sheets are brazed to one another with heat.
Bartels, F., Muschik, T., Gust, W., “Investigations into thermostable microbonds from intermetallic phases,” DVS, (1991), 141, 22-24, reports on a brazing method in which intermetallic phases of binary systems are formed whose components have very different melting points. Examples listed are the binary systems Cu(Sn), Pt(Sn), Ni(Sn), and Ni(In). The report describes the Cu(Sn) system in greater detail.
It is therefore particularly remarkable that despite a great deal of work that has been performed in the field of joining methods to date there has not been any success in satisfying the requirements that were enumerated in the foregoing for manufacturing microstructure components, and that this is so even though microstructure components have already been discussed and produced as promising elements for a number of applications for some time.
Despite the substantial need to employ such components for individual applications, to date it has not been possible to manufacture microstructure components economically in large numbers. One reason for this is that in the past the available joining techniques for bonding the individual component layers were not suitable for manufacturing the microstructure components with sufficient yield. The problem is that the aforesaid requirements cannot be fulfilled satisfactorily. For instance, it is not possible without additional measures to obtain sufficient pressure resistance with sufficient gas- and liquid-tightness both between the microflow channels and to the environment (for instance, He leakage test: 1·10−8 mbar·L/s) and at the same time to ensure that the microchannels remain completely free of the joining agent, for instance a solder/brazing material, and do not become stopped up.